160 A Physical Description 



2. Prismatic Greenstone. — This rock does not differ from 

 the preceding, except that its structure is prismatic, the 

 prisms having from three to seven sides. It is principally 

 seen on Ben Lomond, where prismatic columns are found 

 from three to eight feet in diameter, and sometimes one 

 hundred feet long. There have been no observations on 

 this rock in Tasmania. Similar rocks in New South Wales, 

 Victoria, and Queensland have been microscopically examined. 

 Mr. Allport, of Birmingham, says of that of Gympie (Queens- 

 land) that it is a diorite, containing hornblende triclinic 

 felspar, orthoclase, biotite, and pyrites. There was also a 

 little chlorite and quartz filling up the spaces between the 

 crystals. Many of the crystals were imperfectly crystallised, 

 but the rock was not much altered. 



Some of the diorites of Victoria have formed the subject 

 of a very elaborate paper by Mr. A. W. Howitt, F.G.S., read 

 recently before the Royal Society of Victoria. He says that 

 dykes of diorite are of very frequent occurrence at the 

 Swift's Creek diggings. They are from one to five feet wide, 

 and have not any common direction of strike. They are 

 sometimes visibly crystalline, and composed of white plagio- 

 clase and dark green hornblende. A microscopic examination 

 shows quartz, viridite, apatite, and combinations of iron. 



There can be no doubt that these rocky masses and dykes 

 of diorite wherever they are found are portions of one con- 

 tinued period of volcanic disturbance to which this part of 

 the globe was subject during mesozoic times. In New 

 South Wales and Queensland many of the igneous rocks are 

 intercalated with the carbonaceous deposits. I am not 

 aware that this has been noticed in Tasmania, though the 

 examinations have been very imperfect. But one thing 

 seems to be very certain, and that is that the bulk of the 

 Tasmanian diorites flowed out after the coal period, and 

 probably at the end of the mesozoic epoch alluded to. 

 In a paper on the Hawkesbury sandstone, read before the 

 Royal Society, New South Wales, May, 1882, I have stated 

 my reasons for believing this formation to be an aerial one. 

 It may, therefore, have accumulated round the igneous rocks, 

 and is not in reality under them, though it has that appear- 

 ance. This may be the explanation of the greenstone 

 cappings or outliers. 



Has the island been submerged since the mesozoic period ? 

 On the north side it has — that is, the low-lying portion of the 

 north coast, far away from the table-land. But with regard 



